Method and apparatus for controlling differentially driven wheel slip

Abstract

A slip control system for vehicles having spaced apart differentially driven wheels (200,202) including means (220) for calculating wheel slip according to the ratio of wheel speeds, means (242,230,232) for entering a slip control mode wherein a braking force is applied to the slipping wheel and incrementally varied on a periodic review basis according to recalculated slip values. If slip increases or holds steady, the brake force is incrementally increased. As slip becomes less, the brake force is incrementally reduced, the particular increments of force reduction being selected in accordance with successive measurements of slip. A drive shaft signal (226) is compared to the high wheel speed signal to identify a wheel speed transducer failure.

We claim: 
     
       1. In an anti-slip apparatus for balancing the power transfer between a pair of driven wheels (200,202) when one of the wheels loses traction and rotates faster than the other, said apparatus having first means (212,214) for braking the wheels and second means (216, 222, 220) for generating a slip signal corresponding to any difference in the rotational speeds between the wheels (200, 202), the improvement comprising: third means (230,232, 242) for actuating the first means (212, 214) on the faster rotating wheel (200, 202) to a degree which is variably incrementally modulated in proportion to the difference in rotational speeds between the wheels (200, 202) in response to the slip signal.   
     
     
       2. An apparatus as in claim 1 wherein the third means (230, 232, 242) is modulated by an electrical slip signal developed by the second means (220) and said third means (230, 232, 242) is commanded by said electrical slip signal to incrementally vary the magnitude of the braking force applied to the faster rotating wheel (200, 202), the magnitude of the increments of variation being related to the magnitude of the slip signal. 
     
     
       3. An apparatus as defined in claim 1 further including means (250) operatively connected to the second means (216,222,220) for disabling generation of a slip signal during a service brake application condition. 
     
     
       4. In a system for balancing the power transfer between two differentially driven vehicle wheels (200, 202) when one of said wheels loses traction, said system being of the type which includes means (216, 222) for generating a slip signal representing the speed difference between the wheels and means (212, 214) for applying a braking force to the wheel which loses traction the improvement comprising: means (244) defining a plurality of slip signal value bands each having defined slip value limits,   means (242) repeatedly comparing said slip signal to said bands, and   means (232) variably incrementally changing the braking force according to the slip signal value band to which said slip value corresponds.   
     
     
       5. Apparatus as defined in claim 4 wherein the means (244) further defines both positive and negative incremental changes, each change being associated with a given slip value band and the negative brake force incremental changes being associated with lower slip value bands. 
     
     
       6. Apparatus as defined in claim 4 wherein the means (232, 234) comprises a source (234) of brake fluid pressure and signal responsive valve means (232) connected to said comparing means (242) and responsive to outputs therefrom varying the brake fluid pressure applied to said wheels. 
     
     
       7. Apparatus as defined in claim 6 wherein the means (232) further comprises wheel selection means (230) responsive to outputs from said comparing means (242) to vary the brake fluid pressure to only one of said wheels. 
     
     
       8. Apparatus as defined in claim 4 wherein said comparing means (242) comprises timer means (246) for controlling the periodic repeated comparison of the slip signal value of said bands. 
     
     
       9. Apparatus as defined in claim 4 wherein the means (232) further comprises a variable duty cycle generator (256) connected to receive outputs from the comparing means (242), and being connected to the means (232) for varying the brake fluid pressure applied to said one of said wheels under slip conditions. 
     
     
       10. A method for balancing the power transfer between two differentially driven vehicle wheels (200, 202) when one of said wheels loses traction comprising the steps of: developing a slip signal representing the difference between the rotational speeds of the differentially driven wheels,   applying a brake force to said one wheel to control slip;   defining a plurality of slip signal value bands having defined end values;   repeatedly periodically determining the relationship between the slip signal value and each of said bands, and   incrementally varying the braking force in accordance with an incremental variation value which corresponds to the band within which said slip signal value falls.   
     
     
       11. The method defined in claim 10, including the further steps of: establishing a first high threshold value which prevents the application of a braking force to said one wheel until the slip signal value exceeds a relatively high value and, thereafter, reducing the threshold signal value to maintain braking force until the slip signal is reduced below at least a second relatively low threshold value.